The control of the flow of a liquid usually is gained by restricting the flow in a restriction in some kind of valve, usually a seat slide valve. In such valves it is required, however, a very small area change for providing a very great change of the pressure drop, is the pressure drop varies exponentially with the change of the position of the valve body. When slightly opening seat valves the flow velocity becomes very high and the liquid pressure locally becomes so low that the liquid tends to be vaporized, giving rise to cavitation. In order to avoid this it would be desirable to provide a control device operating, if possible, in an entirely proportional manner.
For the control of flows the use of rotary-dynamic machines is previously known in various connections. Thus, in U.S. Pat. No. 3,709,245 a control valve is disclosed in which a rotor is brought to rotate by means of a whirling rotation of a fluid flow. Energy then can be tapped from the rotor in various ways, for example by the generation of electric current or the like. Due to the inherent characteristics of the rotary-dynamic machines they are, however, not suited for a more exact control of a fluid flow and particularly the pressure thereof.
Displacement machines also have been used as flow meters. Such flow meters also have been used such that the output controls a valve or the like for providing a flow control, see DE-A-2 440 078. Furthermore, it is also known to use a displacement machine having a rotary member mounted in a liquid conduit for controlling the flow and/or the pressure of the liquid by braking the rotary member. In this respect it might be referred to; for example, DE-B-2 840 134, DE-A-3 210 098, FR-A-2 380 585 and U.S. Pat. No. 4,815,278.
Quite surprisingly it has now been found that a particular use of a displacement machine as a pressure control valve in connection with motor vehicle shock absorbers allows the achievement of unexpected advantages hitherto not obtainable with known shock absorber valve structures.